Molecular Orbital Theory: Atomic Structure

Molecular Orbital Theory

To describe the covalent bond formation and nature of electron sharing, two theories have been proposed: Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT). In Valence Bond Theory, only the half filled orbitals of valence shell take part in bond formation and the remaining orbitals retain their identity. But Molecular Orbital Theory (MOT) suggests the combination of all atomic orbitals having comparable energy and proper symmetry. Molecular Orbital Theory (MOT) was developed by F. Hund and R.S Mulliken in 1932. Main postulates of this theory are :

1. Atomic orbitals of comparable energy and proper symmetry combine together to form molecular orbitals.

2. The movement of electrons in a molecular orbital is influenced by all the nuclei of combining atoms. (Molecular orbital is poly centric in nature)

3. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals (AO's) combine together two molecular orbitals (MO's) are formed. One molecular orbital possess higher energy than corresponding atomic orbitals and is called anti bonding molecular orbital (ABMO) and the other has lower energy and is called bonding molecular orbitals (BMO).

4. In molecules electrons are present in molecular orbitals. The electron filling is in accordance with Pauli's exclusion principle, Aufbau principle and Hund's rule.

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Heisenberg's Uncertainty Principle

Definition of Heisenberg's Uncertainty Principle

According to Heisenberg's uncertainty principle, it is not possible to determine precisely both the position and momentum (or velocity) of a moving microscopic particle, simultaneously with accuracy.

Mathematical Expression of Heisenberg's Uncertainty Principle

▵x .▵p => h / 4π

Where  ▵x is uncertainty with regard to the position and ▵p is uncertainty with regard to the momentum of the particle. If ▵x is very small ▵p would be large , that is , uncertainty with regard to momentum will be large. On the other side if we attempt to find out the momentum exactly the uncertainty with regard to position will be large.

Explanation of  Heisenberg's Uncertainty Principle

To determine the position of a small body like electron, it has to be illuminated with electromagnetic radiation. Low energy radiations like ordinary light waves cannot be used to illuminate a small body like electron, since the size of the electron is very small when compared with the wave length of ordinary light. Therefore to irradiate electrons, radiations with shorter wave length are used. When such a high energy radiation is allowed to fall on an electron its velocity changes by a large value. Consequently if we find the position of an electron precisely, there is always an uncertainty in finding the velocity of an electron simultaneously. Thus the determination of position and momentum of a moving electron precisely and simultaneously is impossible.

De-Broglie hypothesis and De-Broglie equation

De-Broglie Hypothesis

In 1924, de-Broglie proposed that matter has a dual character, as wave and as particle.

In Bohr theory, electron is treated as particle. But according to De-Broglie, electron has a dual dual character; both as a material particle and as a wave. He derived an expression for calculating the wave length 'λ' of a particle of mass 'm'  moving with velocity 'v'.

According to this,
wave length = λ =  h / mv  , where 'h' is Planck's constant

This is equation is known as De-Broglie's equation and it is an expression for wave - matter dualism.

The waves associated with particles in motion are called matter waves or De-Broglie waves. They differ from electromagnetic radiations. They have lower velocities, and no electrical and magnetical fields associated with them.

Derivation of  De-Broglie's equation

The de-Broglie's equation can be derived by using the mass energy relationship suggested by Einstein.

 E = mc2
Here 'c' is velocity of light.

Energy of photon  E = hv

∴   hv = mc2

But, v = c /  λ

 ∴  hc / λ  =  mc2

     h / λ  = mc

Hence  λ  =  h / mc

Repalcing 'c' by velocity of a particle 'v'.

     λ  =  h / mv

Since  mv  = p (momentum)

     λ  =  h / p

de-Broglie's wave length of certain particles at 25 o C

Wave length of Electron  60.67 (Ao)
Wave length of Helium atom  0.71 (Ao)
 Wave length of Xenon atom  0.12  (Ao)

properties and uses of Potassium permanganate (KMnO4)

Properties of Potassium permanganate (KMnO4)

1. Potassium permanganate (KMnO4): Action of Heat

Potassium permangante on strong heating gives potassium manganate, manganese dioxide and oxygen.

2 KMnO4 ----------> K2MnO4 + MnO2 + O2

2. Oxidising properties of Potassium permanganate (KMnO4)

Potassium permanganate is a powerful oxidizing agent in alkaline or acidic solution. The relevant half reactions are:

1. Alkaline medium (pH > 7)

MnO4- + 2H2O + 3 e- ----------> MnO2 + 4OH-

2. Acidic medium (pH <7)

MnO4- + 8H+ + 5e- ----------> Mn2+ + 4H2O

A few important oxidizing reactions of Potassium permanganate (KMnO4)

1. In acidic medium potassium permanganate oxidizes green ferrous salts to yellow ferric salts

MnO4- + 8H+ + 5Fe2+ ----------> 5Fe3+ + Mn2+ + 4H2O

2. in acidic medium potassium permanganate oxidizes oxalic acid or oxalate salts to CO2 and water

2 MnO4- + 16H+ + 5 C2O42- -------------> 2 Mn2+ + 10 CO2 + 8 H2O

3. In acidic medium potassium permanganate oxidizes nitrites to nitrate.

2 MnO4- + 6 H+ + 5 NO2- -------------> 2 Mn2+ + 5 NO3- + 3 H2O

4. In acidic medium potassium permanganate oxidises iodides to iodine.

2 MnO4- + 16 H+ + 1 OI- ----------> 2 Mn2+ + 8 H2O + 5 I2

5. In alkaline medium potassium permanganate oxidizes iodides to iodates .

2 MnO4- + H2O + I- ------------> IO3- + 2MnO2 + 2 OH-

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Chemicals in every day life: Dyes

Chemistry in Dyes

Natural dyes are extracted from natural sources. They are used for making coloured fabrics. Probably, the earliest known natural dyes were indigo (a blue dye) and alizarin (a red dye). These were obtained from plants.
Definition of Dyes
A dye is a coloured substance that can be applied in solution or dispersion to a substrate, giving it a coloured appearance.
Usually the substrate is a textile fiber, but it can also be paper, leather, hair, fur, plastic material, wax, a cosmetic base or a foodstuff. Now a days synthetic dyes are used for dyeing purpose.

Classification of Dyes

Dyes are classified either according to their constitution or method of application.

1. Classification of dyes based on Constitution

This classification is based on the distinguishing structural units present in the dye.

1. Azo
2. Nitro
3. Phthalein
4. Triphenyl Methane
5. Indigoid
6. Anthraquinone

2. Classification of dyes based on Application

Depending upon the process of application the dyes are classified as

1. Acid Dyes
2. Basic Dyes

3. Direct dyes
4. Disperse Dyes

5. Fibre reactive dyes
6. Insoluble azo dyes

7. Vat dyes
8. Mordant dyes

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Vat dyes, Mordant dyes

Vat Dyes

Vat dyes are insoluble in water and cannot be used directly for dyeing. But on reduction to a leuco form (colour less), they become soluble in an alkali and acquire affinity for cellulose fibres. A solution of the leuco form can be applied for dyeing or printing. On oxidation the original insoluble dye is formed within th structure of the fibre. Indigo and indigosol O are dyes which belong to this class.

Mordant Dyes

Mordant dyes are primarily used for dyeing of wool in the presence of metal ions. The metal ion binds to the fabric and the dye acting as ligand co-ordinates to the metal ion. The same dye in the presence of different metal ions imparts different colours to the fabrics. The colours imparted by Alizarin in presence of different ions are given below

Ions and colours

Al3+ = Rose red

Ba 2+ = Blue

Cr 3- = Brownish red

Mg 2+ = Violet

Sr 2+ = Red

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Fibre Reactive Dyes and Insoluble Azo Dyes

Fibre Reactive Dyes

Fibre Reactive Dyes attach themselves to the fibre by an irreversible chemical reaction. The dyeing is fast and the colour is retained for a long time. The bonding is through the substitution of leaving group of dye via the hydroxy or amino group of fibres like cotton, wool or silk

Insoluble Azo Dyes

Insoluble Azo Dyes are obtained by coupling phenols, naphthols, arlamines, amino naphthols adsorbed on the surface of a fabric with a diazonium salt. Over 60% of the dyes used are Azo dyes. Cellulose, silk, polyester, nylon, polypropylene, polyurethanes, poly acrylonitriles and leather can be dyed by using these dyes. Azo dyes also find use in cosmetics, drugs, biological stains and as indicators in chemical analysis. Use of such dyes for colouring of food stuffs is not permitted.

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Direct Dyes and Disperse Dyes

Direct Dyes

Direct dyes are water soluble dyes. As the name suggests, these dyes are directly applied to the fabric from aqueous solution and are practically suitable for fabrics like cotton, rayon, wool, silk and nylon which from hydrogen bonds with water. Martius yellow and congo red are important example of this class of dyes.

Disperse Dyes

Disperse Dyes
in the form of minute particles of a suspension diffuse into the fabric, get fixed and impart colour. Such dyes are used for dyeing synthetic fibres like polyesters, nylon and polyacrylo nitrile. Many anthraquinone disperse dyes are suitable for application to synthetic polyamide fibres.

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Acid Dyes and Basic Dyes

Acid Dyes

Acid dyes are usually salts of sulphonic acid and can be applied to wool, silk, polyurethane fibres and nylons. The affinity of acid dyes for nylon is higher than that for other types because polycaprolactum fibers fibers contain a higher proportion of free basic amino groups. Acid dyes do not have affinity for cotton. Orange-1 is a versatile acid dye.

Basic Dyes

Basic dyes contain amino group which in acid form water soluble salts. These dyes get attached to the anionic sites present on the fabrics. Such dyes are used to dye reinforced nylons and polyesters. Aniline yellow and malachite green belong to this class of dyes.

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